1. Field of the Invention
The present invention relates to the fastening of face sheets to honeycomb cores by means of brazing.
2. Description of the Prior Art
The strength to weight requirements of many advanced structures require the use of faced honeycomb panels. While such panels may be formed by means of nonmetallic materials, for certain applications, metal faced honeycombs with metal cores are desirable. Several conventional methods are used to bond the face sheets to the core.
Furnace brazing is a technique where the entire honeycomb panel, with the face sheets abutted thereto, is placed within a furnace. The face sheets are affixed to the core by melting and then solidifying the brazing alloys at the interface. The size of the structure being brazed determines the furnace size and in some applications very large furnaces, with the attendant high equipment costs and difficulties with respect to the maintenance of a proper gaseous atmosphere around the structure during the brazing process, are required. In many applications the brazing process requires a vacuum environment with the attendant cost of large pumps, complex sealing systems and the like. Typically, such equipment requires very large power input. In addition, the thermal cycling times for such large furnaces present the potential for significant problems with respect to the braze alloy attacking the base metal of either the face sheet or the honeycomb core during the long thermal cycling times necessary for proper operation of such equipment.
Some attempt has been made to use infrared heating to braze metal faced honeycomb panels. Very large power requirements are associated with the use of such equipment and proper thermal cycling requires extensive instrumentation and monitoring. In addition, as with the use of a furnace brazing techniques, the control of the vacuum or gaseous atmosphere surrounding the structure is difficult and expensive.
A third means of heating a metal honeycomb structure to braze the face sheets to the core consists of salt bath brazing where the structure is immersed in a molten salt bath. While such a process is feasible, with a number of alloys it is difficult to find salt compositions that do not react with the materials at brazing temperatures. Cleaning of the structures subsequent to such processes is difficult, but necessary to avoid subsequent corrosion problems.
It is the principal object of the present invention to provide a means for forming metal faced honeycomb structures that economically heats the brazed joint with minimal energy consumption. Another object is to minimize the time the panel is exposed to elevate temperatures that may induce detrimental microstructural changes. It is also an object of the invention to eliminate the need for complex vacuum systems that are many times necessary with prior art methods. By using induction heating in the manner set out below, the power requirements for forming such materials are lower without the attendant difficulties in treating large sized panels. Furthermore, the complexity of atmosphere controls is significantly reduced and corrosion problems due to salt residue are non-existent. These objects and other advantages of the invention will be more clearly apparent from the description which follows or may be learned by practice of the invention.